Not a genus I have ever heard of, but Mabberley, The Plant Book, says it's in fam. Rubiaceae and has c. 45 species in trop. America, esp. W Indies; also mentions "febrifugal alks [alkaloids] in bark (Jamaica b.)".

There is an Exostema longiflorum (note spelling, -stema is one of those odd Greek neuters, Latin adjectival epithets take -um to agree). It's native to Cuba and Dominican Republic -- see Tropicos database, link below.

Almost any plants containing alkaloids are potentially poisonous. Even commonplace alkaloids like caffeine and nicotine are lethal in moderately large doses.

Here is a link that might be useful: W3Tropicos